Planned maintenance keeps the machine in good working order throughout, and when a failure does occur brings it back into use quickly. The effectiveness of planned maintenance can therefore be measured by the machine’s availability as it follows from failures and the time taken to repair them — inherent availability — and by its availability as it follows from maintenance and the time maintenance takes — achievement availability.
Why this matters
Measuring planned maintenance answers the question of whether the work has achieved anything. Excellent plant performance cannot come about while the machines are not a hundred percent reliable, and that follows from maintainability which is not a hundred percent either. These two figures therefore measure two sides of the same thing.
The two availabilities
The first availability is the ratio: inherent availability = MTBF ÷ (MTBF + MTTR). MTBF is the mean time between failures — the average period before each failure. MTTR is the mean time to repair — the average period from a failure occurring until the machine is usable again. This ratio therefore states how far the machine is available when reckoned from its failures and the time taken to repair them.
The second is the ratio: achievement availability = MTBM ÷ (MTBM + M). MTBM is the mean time between maintenance — the average period before each maintenance — and M is the average time each maintenance takes. This ratio therefore states how far the machine is available when reckoned from its maintenance and the time that maintenance takes.
The maintenance department’s job does not shrink
Although TPM is maintenance in which everyone takes part, the maintenance department’s duties do not shrink at all. They only change shape — from correction first, prevention second and improvement only occasionally, to prevention first, improvement second, and correction only occasionally.